Natural killer (NK) cells form a first line of defense against virus-infected cells by recognizing alterations in human leukocyte antigen (HLA) expression. NK cells recognize HLA via a panel of killercell immunoglobulin-like receptors (KIRs). Upon binding to HLA, KIRs will transmit inhibitory or activatory signals, with inhibition dominating over activation in a healthy state. As part of the innate immune system, natural killer (NK) cells present a first line of defense against viral infections and tumors. NK cell effector functions, such as cytotoxicity and cytokine release, are controlled by integrated signals from a large panel of both activating and inhibitory receptor. Killercell immunoglobulin-like receptors (KIRs) on NK cells and their ligands, HLA class I molecules, play an essential part in this tight regulation. Both, the KIR gene cluster and the HLA class I loci are known to be extraordinarily diverse, which led to the hypothesis that NK cell immune responses are genetically predetermined to some extent. This is supported by recent epidemiological observations that KIR/HLA compound genotypes with a supposedly activating profile (presence of activating KIR or lack of inhibitory KIRs or their respective ligands) are associated with resistance to HCV (Khakoo et al., Science 2004; 305:872-874) and human immunodeficiency virus (Martin et al., Nat Genet 2002; 31:429-434). [unreadable] [unreadable] Several of these studies take into account HLA-C, the gene locus encoding ligands for KIR2DL receptor, where a functional dimorphism determines KIR specificity. HLA-C group 1 (HLA-C1) alleles, encoding ser77/asp80 of the HLA-Cw a1 domain, bind to the inhibitory receptors KIR2DL2 and KIR2DL3, and probably also to the activating KIR2DS2. In contrast, the HLA-C group 2 (HLA-C2) alleles, encoding asp77/lys80, bind to KIR2DL1 and possibly to KIR2DS1. Homozygosity for HLA-C1 alleles and KIR2DL3 is associated with resolution of HCV infection as compared to homozygosity or heterozygosity for HLA-C2 and KIR2DL1 (Khakoo et al., Science 2004; 305:872-874). To date, the functional mechanisms responsible for these epidemiological associations are poorly defined. It has been proposed that improved resistance to virus infections among KIR2DL3/HLA-C1 positive individuals may be due to weaker NK cell inhibition through KIR2DL3, as compared to NK cell inhibition through KIR2DL1 in KIR2DL1/HLA-C2 positive individuals.[unreadable] [unreadable] In order to characterize NK cell responsiveness in the context of different HLA alleles, we studied the kinetics of NK cell responses in a unique and well-characterized cohort of subjects with distinct KIR/HLA compound genotypes in an influenza A virus (IAV) infection model. NK cells are thought to play an important role early after IAV infection (Liu et al., J Gen Virol 2004; 85:423-428; Draghi et al., J Immunol 2007; 178:2688-2698).[unreadable] [unreadable] and impaired NK cell responses are associated with higher susceptibility to IAV infection in mice (Dong et al., J Infect Dis 2000; 182:391-396). Vaccination with inactivated or attenuated virus induces significant IFN-g responses by NK cells in young children (He et al,, J Virol 2006;80:11756-11766) and NK cells can be directly activated by binding of the influenza hemagglutinin to the NK cytotoxicity receptors NKp44 and NKp46 (Mandelboim et al. Nature 2001; 409:1055-1060). Based on these observations and the lack of a suitable HCV infection model we chose the IAV infection model to analyse NK cell responses in the context of differential KIR/HLA compound genotypes. [unreadable] [unreadable] Using ELISA and multicolor flow cytometry, we demonstrate that bulk NK cells from KIR2DL3+/HLA-C1 homozygous subjects respond faster and release more IFN-gamma to influenza virus infection than NK cells from KIR2DL1+/HLA-C2 homozygous subjects. Subset analysis revealed that those NK cells that bind HLA-C in KIR2DL3+/HLA-C1 homozygous subjects, i.e. NK cells expressing the inhibitory KIR2DL3, were more frequent and responded more rapidly than those NK cells that bind HLA-C in KIR2DL1+/HLA-C2 homozygous subjects, i.e. NK cells expressing the inhibitory KIR2DL1. This differential response depended on the presence of HLA-C on target cells, because there was no intrinsic difference between cytoxicity or IFN-g response of NK cells from HLA-C1 and HLA-C2 homozygous subjects against HLA-C negative targets. Furthermore, the differential response was independent of KIR3DL1/Bw4 interactions. [unreadable] [unreadable] In conclusion, these results represent a functional correlate for the observed epidemiologic associations between KIR/HLA genotypes and outcome of viral infections. In addition, they provide the first evidence for differential innate immune responsiveness at the population level. [unreadable] [unreadable] In collaboration with clinical investigators in NIDDK we have now initiated studies to analyze the NK cell response in patients with HCV and/or HBV, HDV infection.